Is there a way to change the size of the viewport grid in 2.8?
Essentially, no, 2.80 doesn't let us do that any more. The grid was completely rewritten and the option to control its bounds was removed. Having said that, you can reduce the clipping distance, but that just fades out with a gradient instead of having a defined edge, and also clips meshes so it's not really what you want.
If there's strong arguments for getting that brought back, it would be worth writing up a proposal on the community forum (rightclickselect).

There has been some minor discussion of this point there previously, but it seems to have faded away.
eg this discussion:
https://blender.community/c/rightclickselect/nfdbbc/